This invention relates to dielectric compositions and methods of making them.
Capacitors made in accordance with the teachings of U.S. Pat. Nos. 3,604,082--McBrayer et al; 3,659,990--Cato, Jr. et al and 3,988,405--Smith et al include a stack of thin sheets of dielectric material with films of conductive material applied to the sheet to form electrodes. Precious metals such as palladium, platinum or gold have been used for the electrodes. U.S. Pat. No. 3,679,950--Rutt and No. 3,772,748--Rutt describe techniques for applying electrodes of materials other than the precious metals. Compositions having high dielectric constant are advantageous for use in such capacitors. One advantage of a high dielectric constant material is that fewer dielectric layers can be used, thereby providing easier fabrication and reduced cost of electrode material which is a particular consideration when one of the precious metals is used for the electrodes.
Another requirement for capacitors is that the dielectric constant of the material vary only within specified limits with temperature. One E.I.A. (Electronics Industry Association) specification, referred to as X7R requires that the change in dielectric constant be less than .+-.15% over the range -55.degree. C. to 125.degree. C. A similar requirement is stated in the Military BX specification. The commercial Z5U and X5U specifications have less rigorous specifications.
It is an object of the present invention to provide a dielectric material which meets these requirements for use in capacitors.
Barium titanate has long been used as dielectric material in capacitors. Other crystalline compositions exhibiting ferroelectricity or anti-ferroelectricity have been investigated. These include Cd.sub.2 Nb.sub.2 O.sub.7, Pb.sub.2 Nb.sub.2 O.sub.7, Sr.sub.2 Ta.sub.2 O.sub.7 and PbNb.sub.2 O.sub.6. U.S. Pat. No. 2,805,165--Goodman describes binary and ternary lead niobate systems. In column 7, lines 38-45, Goodman refers to a binary barium containing lead metaniobate ceramic which is useful for capacitors. In lines 46-55, Goodman refers to a ternary metaniobate composition in which strontium is substituted in part for lead. In particular, Goodman finds that the compositions having SrNb.sub.2 O.sub.6 between 20-60 mole percent have a high dielectric constant, and compositions containing more than 60 mole percent have a flat temperature-dielectric constant relationship. I have found that while SrNb.sub.2 O.sub.6 does flatten the temperature-dielectric constant relationship, the addition of too much SrNb.sub.2 O.sub.6 lowers the dielectric constant and makes it more difficult to sinter the body. Amounts less than 0.4 mole fraction do not produce a flat curve with a high dielectric constant over the temperature range specified for these capacitors. In accordance with the present invention, the limits on the amount of SrNb.sub.2 O.sub.6 are about 0.4 to 0.5 mole fraction.
It is an object of the present invention to provide ternary niobate compositions in a critical mole fraction range which will meet capacitor specifications and to provide modified ternary niobate compositions which will meet these specifications in a broader range.